Control system



tion relative thereto. A

Patented Jan. 21, 1947 Y CONTROL SYSTEM William H. Formhals, Forest Hills, and George E.

King, Swissvale,

Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application May 30, 1944, Serial No. 538,104 13 Claims. (Cl. 172-239) Our invention relates, in general, to controls for variable'voltage drives gized by a self-excited adjustable voltage generator, and more in particular to variable voltage controls for machine tools that. require a reversing, braking and inching operation;

It is an object of our invention to provide a i drive of the type just mentioned which afiords the performance of reversing, braking and inching operations without necessitating a starting and stopping of the generator at the beginning and end of such operations. Another object of the invention is to provide a variable voltage drive, having a self-excited generator for adjustable output voltage, which sewhose motor is e'nercures a proper building up of the generator voltage even-if the voltage-adjusting rheostat is rated for a wide range of voltage control and set for the lowest voltages of this range.

These and other objects of our invention will .be apparent from the following description of a the circuit diagram of a machine tool feed drive shown in the drawing. a j

In the drawing, numeral tdenotes the base or bed structure of a milling machine. The support 2 for carrying the workpiece 3 is slidably mounted on structure I for horizontal reciprocanoted by 4 is provided for driving the work support 2. The cutter or milling tool is indicated at 5; its driving motor is denoted by 6. The driving gear or transmission 4 is controlled by a motor M with which shaft 1. The armature of drive motor M is 'denoted by 8, an appertaining series field" (compensating) winding by 9 and a shunt field winding by Hi. The motor M derives its electric energization from a generator G whose armature l I is magnetically acted upon by a series field winding l2 (compensating winding) and a shunt field winding l3. The generator shaft I4 is connected with a motor A which is energized through a starting switch IE or other suitable motor starting and control means from an alternating-current line l6 whose. phase conductors are denoted by Li, L2 and L3, ..The motor A may be of any suitable type to secure a substantially constant operating speed.

The connection between generator'G and motor M is controlled'by a number of interconnected control devices and circuits which include two adjustable impedance means, i. e. a generator field rheostat GR and a motor fieldrheostat MR. The generator rheostat GR has a resistor l1 and a contact bar I8 connected in series relation and gear or transmission deit is connected by a ing the running provided with a. slider which is mechanically con-.

When the two sliders are moved in the upward. direction, the efiective resistance of both resisters [1 and I 9 is changed simultaneously. When the slider of rheostat GR reaches the bar [8, no further change occurs in the resistance of the generator shunt field circuit while a further adjustment of the sliders in the upward direction continues to change the resistance in the shunt field circuit of motor -M.

The circuit connections between generator and motor include further an auxiliary current limiting resistor 20 and a dynamic braking resistor 2|. A selector switch SS is provided for select! direction of the motor. This switch has a"forward and reverse position and is under control by the operator. A normall open push contact RB, preferably a push button, serves to start the motor M and-a normally closed push contact SB, such as a push button, is provided for stopping the motor. A push button contact I-B normally open, serves for inching operations. Included in the control system are further a number of contactors or relays denoted by SR, FR, AR, FC, RC, BC, ICR and 20R whose purpose and arrangement will be described presently.

Relay SRserves as a voltage responsive control means. It has its control coil 22 connected across the generator armature ll while its contact 23, when coil 22 is deenergized, short circuits the generator field rheostat GR thus placing the generator field winding i 3 directly across the generator armature. Hy As long as contact 23 is closed, the generator G, when driven at full speed, is in a condition for building its armature voltage up to its maximum value. The relay SR is adjusted sothat it will pick up near the voltage maximum point. This causes'contact 23 to open and to insert the maximum impedance, i. e., the entire resistance of the GR into the circuit of shunt field winding l3, thereby causing the generator armature voltage to drop to its minimum working value. The'dropout value of relay SR is set to be below this minimum voltage value sothat the rheostat GR may 4 control the value of generator output voltage over are adgenerator field rheostat forward" position. lock contact 32 and two main contacts 33 and 44.1 Similarly, relay RC has its coil 35 energized when switch SS is in the reverse position and is procult at 26 is removed and the slider circuit of rheostat GR closed at 21, thereby rendering both rheostats GR and MR efiective in such a manner that the excitation of the generator and motor shunt fields depends on the selected setting of the two respective rheostats. Consequently, the generatorvoltage and, hencepthe running speed of motor M are now dependent on this rheostat setting. Coil 25 of relay FR is energized only during the running operation of the motor but remains deenergized when the motor is at rest as well as during inching operation, as will be more fully explained hereinafter.

Relay AR represents a motor field accelerating relay. It has a control coil 29 connected in series stantially at rest. Similarly, coil 46 is not strong relation with the motor armature 8. Contact of relay AR, when closed, shorts out the motor shunt field rheostat MR whenever the motor armature current reaches a given strength above the rated full load lyalue, for instance, 125% of this value. The relay is further adjusted so that contact 30 opens when the load current drops to r a value slightly above'full load. During acceleration by field weakening, the-relay contact 30 closes and opens a number of times. In other words, the load responsive relay ARacts as a fluttering relay. A

The contactors FC and RC are used for reversing the running direction of motor M and are controlled by'the above-mentioned selector switch SS. Consequently, only one of these contactors is energized at a time. Contactor FC has a coil 3| energized when switch SS is in the illustrated Coil 3| controls an intervided with an mterlock'contact 36 and two main contacts 31 and 38. it will be understood that the reversing contactors FC, and RC and the appertaining selector switch SS (as well as the contactor BC) .could be replaced by a master contrailer of the drumswitch type whose contacts serve'to'directly reverse the polarity ,of the motor armature circuit. Another possibility of reversing the motor is to provide forward and reverse push button'contacts instead of switch SS and contactors FC and RC. These different possibilities of modifying the control scheme are well known. as such, and'hence not illustrated, their 3 specific design does not form part of an invention proper. I The contactor BC serves to control the dynamic braking operation of motor M. Contactor BC is l a single-pole double-throw contactor whose contact assembly is movable between two positions bymeans of two coils/39 and 46, respectively. The assembly is biased toward the illustrated position and coil 46 increases this bias when energized by the counterelectromotive force generated by the armature 8' when the motor is running. C011 39 464s sufliciently deenergized, i. e. the motor sub-- tactor BC which is open at this stage of'operation.

enough to return the assembly into the illustrated dropped ofi position if coil 40 has picked up the assembly and remains energized. The contact assembly of braking contact BC has three contacts denoted by 4|, 42' and 43. The coil 33 is designed to close contacts 4| and'42 and to open contact 43 at the minimum generator voltage. Contact 43, when closed, connects the braking resistor 2| across the motor armature 8. The above-mentioned resistor 20 is series connected with coil 39 and short circuited by the interlock contact 28 of relay FR as long as the latter is in dropped-off condition; Resistor 20 serves to protect-coil 39 of relay BC when the generator voltage is raised to increase the motor speed.

The operation of the drive system as a as follows:

If switch SS is set for forward operation, as

shown in thedrawing, the supply of ,energization to line l6 causes the contactor FC'to be energized from mains LI and L2 through switch SS and coil 3|. Contactor FC closes its main contacts 33 and 34, thereby. preparing the armature and series field circuit of motor M for operation in the forward-running direction. However, armature 8 and field winding 9 remain deenergized because their circuit extends through contact 42 of con- When now motor A is started by closing the starter .l-5, the generator G starts generating an armature voltage tvhichis efiective in coil 22 of relay SR. At first coil 22 is not suficiently energized to open contact 23. Consequently, the shunt field winding l3 of the generator is connected directly across the armature ll while the rheostat GR has minimum or zero impedance and mum working value. Contact 24 of relay SR closes simultaneously i is ineffective.

The shunt field winding III of motor M is also connected across the generator armature, because rheostat MR is shorted out of the load circuit at contact 26. Under these conditions of minimum impedance in both shunt field circuits, the generator voltage builds up toward its maximum. With the-generator at full speed and the generator voltage 'near maximum, relay SR, picks up and, by opening contact 23,

connects the generator field rheostat GR in series with field winding l3. Since the full impedance ofrheostat GR is effective in this circuit, .the generator voltage is now reduced to the miniwith contact 23and prepares an energizing circult for control-relays 8GB and 20R through the closed contact 32 of contactor FC.- However, relays jCRand 20R remain deenergized as long as neither button RB or IB is' depressed. With the generator (3' thus ready for operation, the motor M remains stopped because, as mentioned, its armature circuit is still open atcontact 42 of contactor BC.

, In, order to operate motor M, button RB 'is depressed. This causes control relay lCR to pick up, since its coil is now energized in .a

circuit extending from L2 through the elements at, RB, SB, 32, 24 to Ll. Relay ICR seals itself in at contact 45, so that it stays energized when 1 button RB is subsequently released. Contact 46 serves to move the contact assembly against its 1 bias but functionsin this manner only when coil:

closes and energizes relay 20R through L2, 48, 43; 32, 24, Ll. Contact 49 of relay 20R closes and energizes coil 39 of braking relay BC by generator direct current. Contact 42 of contactor BC-is now closed and completes the armature circuit of motor M so" that the motor is started at the minimum operatingvoltage of the whole generator as determined by the full (maximum) impedance of GR. Interlock contact 4| of contactor BC closes also and energizes coil 25 of relay FR. Contact 26 opens and contact 21 closes, thereby inserting resistor is of rheostat MR into the circuit of motor field windingfld.

Hence, the motor M accelerates to the operating.

scribed above.

In order to stop the motor, the stop button SB is depressed. This breaks the circuit of coil M, the fluttering relay AR is effective as de-Q dd, so that relay 90R drops ofii. This, in turn,

opens the circuit of coil 38 at contact lb, so that relay R is likewise deenergized. The opening of contact ts in relay 20R interruptsthe circuit of coil 353. The coil tit, energized by the counterelectromotive force generated in the motor armature it, causes contact as to close and contacts ti and 62 to open as soon as the motor has nearly stopped. As a result, the armature circuit of the motor is interrupted at contact 52, and coil 2-? of relay m is deenergized due to-the opening of contact 69. At the sam time, the resistor 2! is connected across the motor armature to provide dynamic braking,

With the-motor stopped and the generator in operating condition, as described previously, the actuation of button 13 serves to perform inching operations. The closure of the'contact at button 13 passes current through coil 68 of relay 2GB in circuit L2, 38, IE, 32, 24, Li, so that contact 59 of relay 2GB is closed. Contactor BC picks up through 39, 49, and 28, so that contact A? closes and completes the motor armature circuit. The motor now operates at the minimum running speed as determined by the full impedance of resistors l1 and 19, because relay FR cannot pick up since relay iCR is deenergized H and contact 'il opened. When button IB is released, relay ECR drops out so that the opening of its contact 39 causes the brake contactor BC to open contact 42 and close contact d3, thereby connecting resistor 2| across armature 8 for dynamic braking.

In order to perform the above described operations in the reverse running direction of motor M, switch SS is placed in the re erse position.

In this position, contactorFC re ains deenergized while contactor RC closes contacts 36, 31

and 38, thereby reversing the polarity of the generator current in the circuit of motor armature 8. The other'control connections described in the foregoing remain substantially unchanged except that the contactor RC, its coil and contacts take the place of contactor FC andits corresponding parts.

The energizing coils 44 and 48 0f relays I CR and 20R, respectively, are interlocked wlth relay SR at contact 24. This insures that the generator voltage has built up before the motor M can be operated. If for any reason the generator voltage should collapse while the generator is rurming, relay SR drops out and disconnects the coils of relays ICE and 20R. This, in turn,

causes contactor BC to drop outand to disco n.

nect the motor M from the generator while connecting the dynamic braking resistor 2| across is removed from the generator, and contact 23 of relay SR'short circuits the generator field rheostat GR, so that the generator is in a conditlon for building up and restoring the generator voltage. This, in turn, builds up the motor shunt field and brings the motor to rest. If the generator shunt field circuit opens so that relay derstood by those skilled in the art that other changes can be applied without departing from the spirit and essential features of the invention as set forth in the claims appended hereto.

We claim as our invention:

' 1. A variable voltage drive comprising a gene orator havin an armature and a field winding,

a drive motorand selective control means for reversibly connecting said motor to said armature, selectively adjustable impedance means arranged for COIlIlwClliOi'l with said field winding for varying the excitation of said winding and thereby the working voltage of .said armature within'a given range of voltage values, and voltage responsive relay means connected to said armature for controlling the connection of said impedance means with said winding, said relay means havin a pick-up voltage value above said voltage range and a drop-off voltage value below said range so as to provide higher than working excitation for said winding when the voltage of said armature, while building up, stays below ture, adjustable impedance means arranged for connection with said field winding for varying the excitation of said winding in order to select a working valueof the voltage generated in said armature, within a given voltage range, and voltage responsive relay means connected to said armature for controlling the connection of said impedance means with said winding, said relay means having a pick-up voltage value above said range and a drop-off voltage'value below said range-so as to render said impedance means the motor armature. In this manner, the load inoperative in order to .provide said winding withmore than working excitation when the voltage of said armature, while building up, stays below said pick-up value and when said armature voltage, while decreasing, subsides below said drop-oil value. v

3. A variable voltage drive comprising a generator having an armature forproducing adjustable generator voltage and a field winding ar-- ranged in shunt relation to said armature, a motor connected to said armature for operatin'gat a speed determined by said voltage, adjustable impedance means arranged in series with said field winding for adjusting said voltage within a given range of working voltages, and'voltage responsive relay means connected to said armature for controlling the connection of said impedance means with said winding, said relay means having a pick-up voltage value above said range anda drop-01f voltage value below said range for rendering said impedance means inoperative so as to provide higher than working excitation for said winding when the voltage of said armature, while building up, stays below said pick-up value and age, while decreasing, value;

4. A variable voltage drops below. said dropeofi drive comprising a generator having an armature for producing adjustable generator voltage and a shunt field winding, a drive motor having an armature circuit, motor control means for connecting said armature circuit to said generator armature in order to operate said motor at a speed determined by the value of said voltage, adjustable impedance means in connection with said shunt field winding for controlling its excitation in order to adjust said voltage value, control means responsive to the voltage of said armature for controllin the interconnection of said impedance means and shunt field winding so as to maintain maximum excitation of said shunt field winding when i said armature voltage is below a given building- \up value and to reduce said excitation .to the ivalue determined by the adjustment of said im- 'pedance means after said armature voltage has reached said given value, and interlock -means connecting said voltage responsive control means [with said motor control means so as to prevent operation of said motor before said excitation is 1 reduced by said voltage responsive control means.

5. A variable voltage drive comprising a generator having an armature for producing addustable generator voltage and a shunt field winding, a drive motor having an armature circuit,

motor control means for connecting said armature circuit to said generator armature in order to operate said motor at a speed determined by the value of said voltage, adjustable impedance means in connection with said shunt field winding for controlling'its excitation in order to adjust said voltage value, control means responsive to the voltage of said armature for controlling the interconnection of said impedance means and shunt field winding so as to provide'more than I working excitation for said shunt field winding when said armature voltage is below a given building-up value while reducing, after said armature voltage has reached said given building up value, said excitation to a minimum workin value determined by the maximum impedance value of said impedance means, interlock means connecting said voltage responsive control means with said motor control means so as to prevent operation of said motor before said excitation is reduced by said voltage responsive control means, and circuit means disposed between said impedance means and said motor control means and controlledby the latter for changing said excitation to a working value selected by the adjustnient of said impedance means when said motor is in operation.

6. A variable voltage drive comprising a generator having an armature for producing adjustable generator voltage and a field winding, a motor arranged for operation at a speed determined by: solids-voltage, adjustable impedance means conwhen said armature volt-e tor is inoperation.

voltage, when building up, reaches said pick-up value while establishing maximum impedance after said pick-up value is exceeded and before said generator voltage, when decreasing, subsides below said drop-0E value, and selective motor control means disposed for reversibly energizing said motor by said generator voltage and connectedwith said impedance means for establishing said selected working impedance value when said motor is in operation.

'7. A variable voltage drive comprising a generator having an armature for producing'adjustable generator voltage and a field winding arranged v in shunt relation to said armature, a motor connected to said armature for operating at a speed determined by said voltage, adjustable impedance means arranged in series with said field winding for adjusting said voltage and having selector means for adjusting a working impedance value between a maximum and a minimum impedance value corresponding to respective low and high values of said voltage, voltage responsive relay means connected to said armature for controllin the connection-ofsaid impedance means with said winding and having a pick-up voltage value above said high voltage value and a drop-oil voltage value below said low voltage value for establishing minimum impedance before said generator voltage, when building up, reaches said pickup value and maximum impedance after said pick-up value is exceeded and before said generator voltage, when decreasing,subsides below said drop-oil value, and reversible contact means disposed between said motor and generator for reversibly operating said motor under energizationby said generator voltage and being connected with said impedance means for establishing said selected working impedance value when said mo- 8. A variable voltage drivecomprisinga generator having an armature for producing a justable generator voltage and a field winding arranged in shunt relation to said armature, a motor connected to said armature for operating at r a speed determined by'said voltage, adjustable impedance means arranged in series with said field winding for adjusting said voltage and haviiig selector means for adjusting a working impedance value between a maximum and a minimum impedance value corresponding to respective low and high values of said voltage, voltage responsive relay means connected to said armatilre for controlling the connection of said impedance means with said winding and having a picknectedwith said field winding for adjusting said voltage and having selector means for adjusting a working impedance value between the maximum and a minimum impedance value corresponding to given low and high values respectively of saidrvoltage, voltage responsive relavmeans connected to said armature for controlling the connection of'said impedance means with said winding andhaving a pick-up voltage value above 'said high voltage value and a drop-oil voltage value below said low voltage value for establishingminimum impedance before said generator up voltage value above said high voltage value and a drop-ofi voltage value below said low volt-- age value for establishing minimum impedance before said generator voltage, when building up, reaches said pick-up value and maximum impedance once said pick-up value is exceeded and bein'shunt relation to said armature, a motor connected to said armature for operating at a speed value between a maximum and a minimum impedance value corresponding to respective low and high values of said voltage, voltage responsive relay means connected to said armature for controlling the connection of said impedance means with said winding and having a pick-up voltage value above said high voltage value and a dropofi voltage value below said low voltage value for establishing minimum impedance before said' generator voltage, when building up, reaches said pick-up value and maximum impedance once said pick-up value is reached and before said generator voltage, when decreasing, subsides below said drop-oil value, motor control means disposed for energizing said motor by said generator voltage and connected with said impedance means for establishing. said selected working impedance value when said motor is caused to operate by said motor control means, and inching control means disposed for energizing said motor by said generator voltage and connected with said impedance means so as to establish said maximum impedance value during inching operations.

10. A variable voltage drive comprising a generator having an armature for producing adjustable generator voltage and a field winding, a motor connected to said armature for operating at a speed determined by said voltage, adjustable impedance means connected with said field winding for adjusting said voltage and having selector means for adjusting a working impedance value between a maximum and a minimum impedance value corresponding to respectivelow and high values of said voltage, voltage responsive relay means connected to said armature for controlling the connection of said impedance means withsaid winding and having a pick-up voltage value above said high voltage value and a drop-oir'voltage value below said low voltage value for establishing minimum impedance be- ,fore. ,said generator voltage, when building up,

reaches said pick-up value and maximum impedance once said pick-up value is reached and before said generator voltage, whendecreasing, subsides below said drop-off value, motor control means disposed for energizing said motor by said generator voltage and connected with ,said imp'edance means for establishing said selected 1 working impedance. value when said motor is tive low' and high values of said voltage, voltage responsive relay means connected to said armature for controlling the connection-oi said impedance means with said winding and having a pick-up voltage value above said high voltage value and a drop-on voltage value below said low voltage value for establishing minimum impedance before said generator voltage, when building up, reaches said pick-up value and after said generator voltage, when decreasing, subsides below said drop-off value, motor control means disposed for energizing said motor by said generator voltage and connected with saidv impedance means for establishing said selected.

working impedance valuewhen said motor is in operation, said motor control means being interlocked by said voltage responsive control means so as to permit operating said motor only whensaid latter control means are in picked-up condition, dynamic braking means including a resistor, and means under control by said motorcontrol means for connecting said resistor in parallel to said motor only when said motor control means are in inoperative condition.

12. A variable voltage drive comprising a generator having an armature for producing ad justable generator voltage and a field winding, a motor connected to said armature for operating at a speed determined by said voltage, adjustable impedance means connected with said field winding for adjusting said voltage and hav-- ing selector means for adjusting a working im-.-

pedance value between a maximum and a mini-' mum impedance thereafter and before said generator voltage, when decreasing, subsides below said drop-off value, motor control means disposed for energizing said motor by said generator voltage and connected with said. impedance -means for establishing said selected working impedance value when said motor is caused to operate by said motor control means, inching control means disposed for energizing said motor iby said generator voltage and connected with I caused tooperate by said motor control'means, 9

inching control means disposed for energizing said motor by said generator voltage and connected with said impedance means so aslto establish said maximum impedance value during inching operations, and interlock means controlled by said voltage responsive control means and connected with said motor control means and inching control means for preventing the two latter means from operating; said motor when said voltage responsive control means are in dropped-air condition.

11. A variable voltage drive comprising a generator having an armature for producing adjustable generator voltage and a field winding,

a motor connected to said-armature for operating at a speed determined bysaid voltage, adjustable impedance means connected with said field winding for adjusting said voltage and having selector means for adjusting a working impedance value between a maximum and a minimum'impedance' value corresponding to respecsaid impedance means so as to establish said maximum impedance value during inching operations, interlock means controlled by said volt-' age'responsive control means and connected with said motor control means and inching control means for preventing the two latter means from operating said motor when said voltage responsive control means are in dropped-oi condition, a dynamic braking resistor connected in pafallel to said motor, and means under control by said motor control means and said inching control means for disconnecting said resistor when said motor is energized by either of said latter two means.

13. A variable voltage drive comprising a generator having an armature for producing adjustable generator voltage and a shunt field winding, a drive motor having an armature and a shunt field winding disposed for connection to said generator, two adjustable resistance devices connected with said respective shunt field wind- .ings' and having interconnected adjusting mema a 11 bers respectiyely for simultaneously adjusting the excitation of said field windings in accordance with aflselected motor speed, a voltage responsive relay connected across saidgenerator armature for controlling the connection of said generator field winding with said appertaining resistance device so as to short circuit said latter device during starting operation of said generator, another relay connected between said generator and said motor for shorting saidother de- 10 vice, and reversible motor control means disbosed ior reversibly energizing said motor by saidgenerator voltage and connected with said resistance devices so as to operate said motor at said 5 selected speed, said other relay being connected 

